Currently, products that conform to the IEEE 802.11b/g/n wireless Local Area Network (LAN) standards and use the 2.4 GHz Industry-Science-Medical (ISM) band are becoming widespread as infrastructure in home networks. In Japan, the ISM band is a frequency bandwidth open for unlicensed use at powers equal to or less than 10 mW. In addition to wireless LAN standards, other communications systems such as ZigBee, Bluetooth, and cordless telephones also use the ISM band. As a result, signals from other communications systems sometimes interfere with wireless LAN communication. Artificial noise emitted from electrical appliances that use high-frequency devices, such as microwave ovens, is also known to appear in the ISM band.
Other than wireless LANs, a Power Line Communication (PLC) system using power lines as a communication medium are a known form of infrastructure in home networks. A PLC system communicates by impressing a communications signal on a power line at a low or high frequency not being used for power transfer. However, these frequency bands are subject to interference from a plurality of communications devices that perform PLC with different communication methods, noise produced by electronic devices connected to the power line, broadcast waves from broadcasting stations, electric radiation leaked from external electronic devices, and the like.
As a result, the channel between a transmission device and a reception device in a wireless communication system or a PLC system that use the ISM band is greatly influenced by artificial noise and interference from other systems. In the present description, interference from other systems, artificial noise from electronic devices, and the like are collectively referred to as external noise. Typically, external noise is characterized by having a large power level and by appearing at a specific time and a specific frequency. Therefore, a reception device designed with the assumption of communicating in an environment of Additive White Gaussian Noise (AWGN) produced at the reception device cannot perform optimal reception in an environment in which external noise exists, and bit error rate characteristics degrade. As a result, the throughput of the communications system decreases.
To address the above problem, the question of optimal reception in an environment of external noise, such as impulse noise, has been studied. For example, Non-Patent Literature 1 discloses a method for optimal reception that estimates necessary parameters when using a Middleton class A impulsive radio noise model as a statistical model for external noise. However, with the method disclosed in Non-Patent Literature 1, additional calculation for estimation of parameters for external noise is necessary.
Non-Patent Literature 2 proposes a simpler method that estimates a symbol on which external noise is imposed and sets the log likelihood ratio (LLR) of bits included in the symbol to zero during demodulation and decoding. With this method, when calculating the LLR to input into the decoder, if the LLR for a bit is larger than a predetermined threshold, the bit is determined to have been influenced by powerful external noise, and the LLR is set to 0 for decoding. In this way, reception in an external noise environment is achieved with a relatively simple approach.